Steel plants produce steel sheets of various thicknesses and dimensions through hot rolling processes. In the hot rolling process, a thin steel sheet called a coil is produced from a thick steel plate called a slab by rolling out the slab held from above and below by a set of work rolls of a rolling mill. The surface quality of a coil thus produced depends on the surface condition of the work rolls. In addition, rolling a plurality of thick plates one after another using a certain set of work rolls gradually deteriorates the surface condition of the work rolls. For this reason, the rolling of a steel plate requiring a high quality is preferably done with a set of work rolls while the rolls are still fresh.
Moreover, rolling a steel plate may sometimes leave a groove as wide as the plate on the surface of the work rolls. Accordingly, the groove left on the surface of the work rolls by a narrower steel plate rolled earlier may sometimes leave a flaw on the surface of a wider steel plate that is rolled later. In addition, the specification of rolling equipment or the like limits the difference in thickness between successively rolled two steel plates to a certain range. Furthermore, especially thinner steel plates cannot be successively rolled in large numbers, in order to prevent the deterioration in the durability of work rolls. In this regard, a technique for determining the sequence of rolling steel plates has been used in order to maintain the quality of steel sheets and to improve the productivity while satisfying various constraints as described above. The technique is described in an article entitled “Hot Mill Scheduling,” Tokyo Research Laboratory, IBM Japan, Ltd., which can be viewed on the research.ibm.com web site under projects/optimization (31 Jan. 2007).
In the foregoing technique, a method termed as local improvement is used in order to further improve a draft rolling sequence. The local improvement process is performed by firstly introducing an evaluation value indicating the efficiency of processing steel sheets. Here, this evaluation value increases as the number and the total length of steel sheets that can be processed by one set of work rolls increase, or as the number of high-quality steel sheets that can be processed by one set of work rolls increases. Then, a judgment is made as to whether or not the evaluation value increases when another certain steel plate is inserted into an array of steel plates already arranged in a processing sequence. When the evaluation value increases, the certain steel plate is inserted and the processing is performed on the new array of steel plates in the new sequence. The degree of increase of the evaluation value may differ depending on the position into which the certain steel plate is inserted. In this case, the steel plate is inserted into the position providing the highest degree of increase of the evaluation value. All the steel plates to be processed are examined in this insertion process.
When the insertion position is determined only by using the evaluation value in the local improvement process, the processing sequence thus determined is not efficient in some cases. This is because a steel plate inserted at an earlier stage of the local improvement inhibits the constraints from being satisfied by other steel plates to be inserted at later stages, and accordingly makes it difficult to insert the other steel plates at the later stages. For example, assume that there is a constraint that the position of a certain type of steel plate in the processing sequence must be no greater than a certain upper limit number. On this assumption, once a new steel plate of the certain type is inserted into a position near the upper limit number, it becomes difficult to insert another steel plate before the new steel plate. In this way, when the local improvement is performed only by using the evaluation value indicating the degree of processing efficiency, the local improvement may eventually determine the inefficient processing sequence in some cases.
In this regard, an object of the present invention is to provide a system, a method and a program which are capable of solving the above-mentioned problem. This object is achieved by combining the features recited in the independent claims in the scope of claims. In addition, the dependent claims define more advantageous specific examples of the present invention.